The sources of noise in a vehicle are many and include, among others, power train, driveline, tire contact patch (excited by the road surface), brakes, and wind. The noise generated by all these sources inside the vehicle's cabin can cover a rather large frequency range that, for normal diesel and petrol vehicles, may go up to 6.3 kHz (above this frequency, the acoustical power radiated by the noise sources in a vehicle is generally negligible). Vehicle noise is generally divided into low, middle and high frequency noise. Typically, low frequency noise may be considered to cover the frequency range between 50 Hz and 500 Hz and may be dominated by “structure-borne” noise: vibration is transmitted to the panels surrounding the passengers' cabin via a variety of structural paths and such panels then radiate noise into the cabin itself. On the other hand, high-frequency noise may typically be considered to cover the frequency range above 2 kHz. High-frequency noise is typically dominated by “airborne” noise: in this case the transmission of vibration to the panels surrounding the passengers' cabin takes place through airborne paths. It is recognized that a grey area exists, where the two effects are combined and neither of the two dominates. However, for passenger comfort, it may be important that the noise is attenuated in the middle frequency range as well as in the low and high frequency ranges.
For noise attenuation in vehicles, such as cars and trucks, the use of insulators, dampers and absorbers to reflect and dissipate sound and thus reduce the overall interior sound level is well known.
Insulation is traditionally obtained by means of a “mass-spring” barrier system, whereby the mass element is formed by a layer of high density impervious material normally designated as heavy layer and the spring element is formed by a layer of low density material like a non compressed felt or foam. The name “mass-spring” is commonly used to define a barrier system that provides sound insulation through the combination of two elements, called “mass” and “spring”. A part or a device is said to work as a “mass-spring” if its physical behaviour can be represented by the combination of a mass element and a spring element. An ideal mass-spring system acts as a sound insulator due mainly to the mechanical characteristics of its elements, which are bonded together.
A mass-spring system for sound insulation in a vehicle is normally placed on top of the steel layer, with the spring material in contact with the steel. If considered as a whole, the complete system (the mass-spring plus the steel layer) may have the characteristic of a double partition. The insertion loss is a quantity describing the effectiveness of the mass-spring system when put on top of the steel layer, independently from the insulation provided by steel layer itself. The insertion loss thus shows the insulation performance of the mass-spring system.
The theoretical insertion loss (IL) curve (measured in dB) that characterizes a mass-spring system will now be described. On most of the frequency range, the curve increases with the frequency in an approximately linear way, and the rate of growth is about 12 dB/octave. This linear trend is considered effective for insulating against the incoming sound waves. For this reason, mass-spring systems have been widely used in the automotive industry. This trend is achieved only above a certain frequency value, called “resonance frequency of the mass-spring system,” at which the system is not effective as a sound insulator. The resonance frequency depends on the weight of the mass element (the higher the weight, the lower the resonance frequency) and on the stiffness of the spring (the higher the stiffness, the higher the resonance frequency). At the resonance frequency of the mass-spring system, the spring element transmits the vibration of the underlying structure to the mass element in a very efficient way. At this frequency, the vibration of the mass element is even higher than that of the underlying structure, and thus the noise radiated by the mass element is even higher than the one that would be radiated by the underlying structure without mass-spring system. As a consequence, the IL curve has a negative minimum around the resonance frequency.
Both absorbing and insulating systems on their own have only a small bandwidth of frequencies where they work optimally. The absorber generally works better in the high frequencies, while the insulator generally works better in the low frequencies. Furthermore, both systems are sub-optimal for use in a modern vehicle. The effectiveness of the insulator may strongly depend on its weight: e.g., the higher the weight, the more effective the insulator. The effectiveness of the absorber, on the other hand, may strongly depend on the thickness of the material: e.g., the thicker the better. Both thickness and weight are becoming increasingly restricted, however. The space in a car where the trims are placed is also restricted. For example, the weight impacts the vehicle's fuel economy and the thickness of the material impacts the vehicle's spaciousness.
Recently, a trend towards lower weights for the mass layer or heavy layer for conventional mass-spring systems has decreased the average weight from about 3 (kg/m2) to around 2 (kg/m2). This drop in area weight also means using less material and thus less cost. Even lower weights down to 1 (kg/m2) are possible and present on the market, but the technology to achieve this may be expensive and may have drawbacks in particular for low volume mass production. Typical mass layers are made of highly-filled dense materials, such as EPDM, EVA, PU, PP, etc. Since these materials have a high density (normally above 1000 (kg/m3)), it may be necessary to make a very thin layer to obtain the low area weight. However, this can increase production costs and cause production problems, such as the material tearing easily during molding.
The insulation performance of an acoustical barrier is typically assessed by sound transmission loss (TL). The ability of an acoustical barrier to reduce the intensity of the noise being transmitted depends, at least in part, on the nature of the material(s) forming the barrier. An important physical property controlling sound TL of an acoustical barrier is the mass per unit area of its component layers. For best insulating performance, the mass layer of a mass-spring system will often have a smooth high-density surface to maximize reflection of noise waves, a non-porous structure and a certain material stiffness to minimize vibration. From this viewpoint, it is known that many textile fabrics, that are either thin or porous in structure, are not ideal for noise insulation.
JP 2001310672 discloses a multi-layer structure consisting of two absorbing layers with a sound reflecting film layer in between. The film layer reflects sound penetrating the absorbing layer back to the same absorbing layer, thereby increasing the absorbing effect of the multilayer structure. The system may be tuned by optimizing the film thickness and the density of the film.
JP 2001347899 discloses a common mass-spring system with an additional absorbing layer on top of the mass layer. Because of the increase in noise attenuation guaranteed by the additional absorbing layer, the thickness and/or the density of the mass layer may be reduced.
EP 1428656 discloses a multi-layer structure consisting of a foam layer and a fibrous layer with a film in between both layers. The fibrous layer, made of compressed felt, may function as an absorbing layer with an airflow resistance (AFR) of between 500 and 2500 (Nsm−3) and an area mass of between 200 and 1600 (g/m2). The disclosed foam layer has a low compression force deflection with stiffness between 100 and 100000 (Pa), comparable to the stiffness of a felt layer normally used as a decoupler. The film used is preferably perforated or thin enough to not have an impact on the absorption of both absorbing layers together. The film is called acoustically transparent to indicate that the sound waves may pass the film. The thickness disclosed is in the range of 0.01 (mm) or less for this purpose.
Normally, to reduce the sound pressure level in the passengers' compartment, a vehicle requires a good balance of the insulation and absorption provided by the acoustical trim parts. The different parts may have different functions (e.g., insulation may be provided on the vehicle's dash, while absorption may be provided on the carpet). There is a current trend, however, to refine the acoustical functions of particular areas of the vehicle, as part of optimizing the vehicle's overall acoustical performance. As an example, the vehicle's inner dash may be split in two parts, one providing high absorption and another providing high insulation. Generally, the lower part of the dash may be more suitable for insulation, because the noise coming from the engine and the front wheels through this lower area is more relevant, while the upper part of the dash may be more suitable for absorption, because some insulation may already be provided by other elements of the car, for instance the instrumentation panel. In addition, the backside of the instrumentation panel may reflect sound waves coming through the part of the upper dash hidden behind the instrumentation panel itself. These reflected sound waves could be effectively eliminated using absorbing material. Similar considerations may be applied to other acoustical parts of the car. For instance, insulation is typically used in the foot-well areas and around the tunnel area, while absorption is typically used underneath the front seat and in the rear floor panels.
For the above reasons, vehicle manufacturers typically use patches or locally applied additional material. For instance, U.S. Pat. No. 5,922,265 discloses a method of applying heavy layer material in specified areas of a trim part, while the areas without the heavy layer material will act as absorber. These hybrid type of products can have the disadvantage that they still increase the area weight to obtain a combined noise absorbing and insulating solution. They can also be labor and cost intensive. In addition, material used as a decoupler for an acoustic mass-spring system may not be optimal for use as an absorber. Furthermore, the use of different types of materials can make recycling of the parts and discarded material more difficult.